Editing Atari 520ST+

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{{Infobox computer
| name = Atari 520ST+
| image = [[File:Atari_520ST_Plus.jpg|250px]]
| manufacturer = Atari Corporation
| type = Home computer / personal computer
| caption = Atari 520ST+ with SM124 monochrome monitor
| release date = {{Start date|1986}}
| discontinued = 1987 (superseded by STF/STFM variants)
| price = Approximately US$599–US$799 (varied by bundle and market)
| cpu = Motorola MC68000 @ 8 MHz
| memory = 1 MB RAM
| storage = External 3.5-inch floppy drive (Atari SF354 SSDD 360 KB or SF314 DSDD 720 KB); external ACSI hard disks
| display = 320×200 (16 colours), 640×200 (4 colours), 640×400 (mono)
| sound = Yamaha YM2149F (3 voices + noise)
| dimensions = 70 mm × 476 mm × 241 mm (2.75 × 18.75 × 9.5 in)
| weight = ≈4.3 kg (9.5 lb)
| os = Atari TOS 1.00 / 1.02 (192 KB ROM)
| predecessor = [[Atari 520ST]]
| successor = [[Atari 520STFM]]
}}

The '''Atari 520ST+''' (sometimes written '''520ST Plus''') is a variant of the Atari [[Atari 520ST|520ST]] home computer that shipped with 1 MB of RAM instead of the standard 512 KB. Introduced in 1986, it was created to differentiate the higher-memory version from the base 520ST, particularly after the [[Atari 1040ST|1040STF]] established 1 MB as the standard for the ST line.

The 520ST+ retained the original "short-case" form factor with an external power supply and no internal floppy drive, making it physically identical to the standard 520ST. The "+" suffix was Atari's way of indicating the doubled RAM without changing the model number entirely. It was sold primarily in European markets.

== History ==

When the ST line launched in 1985, the 520ST shipped with 512 KB of RAM and the [[Atari 260ST|260ST]] had 256 KB. By 1986, Atari had introduced the 1040STF with 1 MB of RAM and an internal floppy drive, at a price point that made the 512 KB 520ST look less competitive.

Rather than discontinue the short-case design entirely, Atari upgraded some 520ST units to 1 MB of RAM and marketed them as the "520ST+". This allowed retailers to offer a 1 MB machine at a lower price point than the 1040STF (which included the internal floppy drive and, in some models, an internal power supply).

The 520ST+ was not a distinct hardware design — it used the same motherboard as the 520ST with additional or different RAM chips installed to provide 1 MB. The naming convention simply distinguished it from the 512 KB models in dealer and consumer documentation.

== Architecture ==

The 520ST+ is architecturally identical to the [[Atari 520ST]], differing only in RAM capacity:

* '''CPU:''' Motorola MC68000P8 or MC68000FN8 @ 8.00 MHz
* '''Architecture:''' 16/32-bit (32-bit internal, 16-bit external data bus, 24-bit address bus)
* '''RAM:''' 1 MB, using 16× 256k×4 or equivalent DRAM configuration
* '''ROM:''' 192 KB TOS (TOS 1.00 in 6× 32 KB ROMs, or TOS 1.02 in 2× 128 KB ROMs on later units)
* '''Custom ICs:''' Shifter (C025914), GLUE (C025915), MMU (C025912), DMA (C025913) — identical to standard ST

=== Major ICs ===

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{| class="wikitable"
! Function !! Part Number !! Notes
|-
| CPU || MC68000P8 / MC68000FN8 || 8 MHz, DIP-64 or PLCC-68
|-
| Video Shifter || C025914-38 || Pixel serialiser
|-
| GLUE || C025915-38A || Bus timing, address decode
|-
| MMU || C025912-38 / C100109 || DRAM controller; later revision on some boards
|-
| DMA Controller || C025913-38 || ACSI/floppy DMA
|-
| FDC || WD1772-00-02 || Floppy disk controller
|-
| MFP || MC68901 || Timers, interrupts, UART
|-
| ACIA (×2) || MC6850 || Keyboard/IKBD and MIDI
|-
| Sound/PIO || YM2149F || 3-voice PSG + I/O
|-
| RS-232 drivers || MC1488 / MC1489 || Serial port level conversion
|}

== Connectors ==

Identical to the [[Atari 520ST]]:

* '''Video/Monitor:''' 13-pin DIN (analog RGB, mono, sync, audio)
* '''Serial (Modem):''' DB-25 male, RS-232C
* '''Parallel (Printer):''' DB-25 female
* '''MIDI In/Out:''' Two 5-pin DIN connectors
* '''External Floppy:''' 14-pin DIN
* '''ACSI (Hard Disk):''' DB-19 female
* '''Cartridge:''' 40-pin edge connector
* '''Joystick/Mouse:''' Two DE-9 male ports
* '''Power:''' 7-pin DIN (external PSU: +5V, +12V, −12V)

== Display ==

Same three display modes as all ST-family machines:

* '''Low resolution:''' 320×200, 16 colours from 512-colour palette, ~50/60 Hz
* '''Medium resolution:''' 640×200, 4 colours, ~50/60 Hz
* '''High resolution (mono):''' 640×400, monochrome, ~71.2 Hz (requires SM124/SM125)

== Sound ==

* '''PSG:''' Yamaha YM2149F — 3 square-wave channels + noise, 5-bit envelope
* '''MIDI:''' Built-in MIDI In/Out at 31.25 kbaud
* '''Audio output:''' Mono line-level on monitor DIN connector

== Comparison with Other ST Models ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable"
! Feature !! [[Atari 260ST|260ST]] !! [[Atari 520ST|520ST]] !! '''520ST+''' !! [[Atari 520STFM|520STFM]] !! [[Atari 1040STF|1040STF]]
|-
| RAM || 256 KB || 512 KB || '''1 MB''' || 512 KB / 1 MB || 1 MB
|-
| Internal Floppy || No || No || '''No''' || Yes (DSDD) || Yes (DSDD)
|-
| RF Modulator || No || No || '''No''' || Yes || No
|-
| Internal PSU || No || No || '''No''' || Yes || Yes
|-
| Case Style || Short || Short || '''Short''' || Long || Long
|}

== Service and Repair ==

The 520ST+ shares all service procedures with the standard ST family:

* [[Atari ST General Maintenance]]
* [[Atari ST Troubleshooting Guide]]
* [[Atari ST Capacitor Replacement Guide]]

== See Also ==

* [[Atari 260ST]] — 256 KB entry-level model
* [[Atari 520ST]] — Standard 512 KB model
* [[Atari 520STM]] — 520ST with RF modulator
* [[Atari 520STFM]] — 520ST with floppy and RF modulator
* [[Atari 1040ST]] — 1 MB with internal floppy

== References ==
<references/>

{{Navbox-AtariVintageComputers}}

[[Category:Atari Systems]]